1. Field of Invention
The invention relates to an optical head having a diffuser structure, and more particularly to an optical head having a tube-like or rod-like diffuser structure for improving the success rate of reading two-dimensional bar codes and eliminating the problem of dead spots of recognition caused by reflected stray light.
2. Related Art
FIG. 1 is a schematic illustration showing application of a conventional optical head 101. Referring to FIG. 1, the optical head 101 for reading an object 102 includes a hollow housing 110, a light source module 120 and a light sensing module 140. The light source module 120 is fixed in the hollow housing 110 and provides a light ray. The light ray is reflected in the hollow housing 110 and then illuminates the object 102. The light sensing module 140 fixed in the hollow housing 110 senses the light ray reflected by the object 102.
The light ray reflection may include specular reflection, spread reflection and diffuse reflection. The specular reflection represents that the incident angle of the light ray is the same as the angle of reflection of the light ray. The spread reflection occurs on an uneven surface. When the number of angles of reflected light rays exceeds the value, the angles of reflection of some reflected light rays are the same as the incident angle. The diffuse reflection occurs on a rough or uneven surface, and the reflected light rays have different angles.
Because the inner wall surface of the hollow housing 110 does not pertain to a specular surface, stray light L10 may enter the light sensing module 140. The currently used light sensing module 140 includes a CMOS (Complementary Metal Oxide Semiconductor) sensor having the resolution of about 100*100 DPI. The field of view (FOV) of the lens in the CMOS sensor of the light sensing module 140 is about ArcTan(pattern diagonal/2/EFL)*2. The light source module 120 is usually a light-emitting diode (LED) for emitting infrared light with the wavelength of about 800 to 950 nm. A lens fixing structure of the light sensing module 140 may adjust the image acquiring distance from the lens to the object 102. The illumination effect of the optical head 101 on the object 102 is controlled by adjusting the lens fixing structure of the light sensing module 140, designing the reflection property of the inner wall surface of the hollow housing 110 and adjusting the position of the light source module 120.
For example, the resolution of the lens is about 50 to 80 lp/mm, wherein RI>80%, and the f-number (F/N) cannot be too high to increase the illumination loading. The allowable tilt angle of the optical head for reading the object has to reach 30 to 40 degrees, and the depth of field of the lens is greater than ±0.7 mm. The object 102 may be an e-book on which two-dimensional bar code patterns are printed. Because the surface material of the sheet of the e-book can easily reflect light and the tilt angle A between the optical head and the surface of the sheet may reach 0 to 30 degrees or even 0 to 40 degrees when the user holds the optical head to read the e-book. Thus, it is necessary to prevent the strong light reflected by the surface of the sheet from entering the light sensing module 140 so that the object cannot be recognized.
In FIG. 1, the light ray outputted from the light source module 120 of the LED, for example, impinges on the inner wall surface of the hollow housing 110 of the conical pen head to increase the intensity of the incident light. However, there is the reflected light still can enter the light sensing module 140 when the incident angle is equal to 24 degrees. In addition, the inner wall surface (typically referred to as a reflective shell) having the reflecting effect also makes the stray light travel back to the light sensing module 140 so that the contrast is further decreased.
FIG. 2 is a schematic illustration showing application of another conventional optical head. As shown in FIG. 2, the optical head is similar to that of FIG. 1 except that the light ray of a LED 120 directly illuminates the object 102. Such a condition tends to make the reflected light enter the light sensing module 140 so that the contrast is further decreased.
Consequently, the drawback of the conventional optical head is that the dead spots of recognition caused by the reflected strong light cannot be eliminated so that the illumination efficiency cannot be effectively increased, the usage and power consumption of the LED cannot be decreased, and the allowable range of the tilt angle cannot be effectively broadened.